High reliability semiconductor packages must have the exposed metal parts highly resistant to salt and corrosive atmospheres. If appreciable corrosion occurs, in time, the lid of the package will be penetrated and will lose its hermeticity. In addition, the corrosion will destroy the identification markings on the lid of the package.
Conditions for testing the package for corrosion are defined by MIL-STD 883C, Method 1009.4, Test Condition A. Plating of themetal parts of the package is governed by MIL-M-38510G which states that a single layer of nickel between 50 and 350 microinches, and a single layer of gold having a thickness of 50 to 225 microinches is used. Alternatively, a multi-layer structure of pure nickel and pure gold may be used where the sum of the thicknesses of the nickel layers is between 50 and 350 microinches and the sum of the thicknesses of the gold layers is between 50 and 225 microinches. The gold should have a purity of 99.7%, or better.
Corrosion occurs on the metal parts because of defects in the nickel and gold layers of plating. The defects may be induced by imperfections in the base metal--either Alloy F15 also known under the registered trademark Kovar, or Alloy 42. This defect is usually caused by some type of contamination such as alumina embedded in the metal surface. It is characteristic of both thin layers of nickel and gold that they will not bridge over the contaminating particle. The imperfection may also come about from contamination particles existing in the plating solutions. These particles plate out with the nickel and gold layers and leave microscopic channels in the protective plating. A tiny channel in the nickel layer will not be bridged by the outside gold layer and thereby leave a microscopic hole extending to the surface of the base metal.
Indications of corrosion are visually detected by a deposit of iron oxide surrounding the defect hole. In the MIL-STD 883C salt atmosphere corrosion test the chloride ion in the salt solution greatly accelerates the corrosion reaction. Since iron has a normal electrode potential of -0.440 volts in the electromotive series and gold has a normal electrode potential of +1.4 volts, diffusion of iron ions is accelerated to and through the gold layer where the iron oxide deposits around the hole in the gold layer. If the nickel layer had no defect in coincidence with the gold layer, corrosion would not take place.
The referenced patents, and especially U.S. Pat. No. 4,601,958, by the inventor hereof U.S. Pat. No. 4,666,796 is based on a Continuation application of the foregoing patent described a plating process and method for plated metal parts, suitable as sealing lids for semiconductor packages, in which an iron based alloy substrate layer is electroplated with a first layer of nickel; a first layer of gold then is electroplated on the first nickel layer. A second layer of nickel is electroplated over the first gold layer, and a second layer of gold is then electroplated over the second layer of nickel. Thus, four layers are sequentially electroplated on the substrate layer. Surprisingly, the resultant plated parts with the multiple nickel-gold layer sets exhibited higher salt atmosphere corrosion resistance than parts plated with a single nickel-gold set although, due to thinner layers, the overall use of gold could be reduced.